Method and apparatus for acquiring a ranging signal of a positioning system

Abstract

A method and corresponding apparatus for accurately determining the offset of the carrier frequency of a received signal from a nominal frequency (the offset due to for example Doppler shifting), such as is done in a ranging receiver when acquiring or tracking a signal transmitted by a beacon (such as a satellite) of a positioning system. The method amplifies a conventional correlation by performing a special noncoherent integration of the real and imaginary components of the output of a conventional (coherent) correlation calculation, resulting in a complex phasor having a phase that bears information about the offset of the carrier frequency from the nominal carrier frequency.

Description

FIELD OF THE INVENTION[0001]The present invention relates to CDMA (Code Division Multiple Access) spread spectrum receivers, and more particularly to fast acquisition GPS (Global Positioning System) receivers (or, more generally, ranging receivers).BACKGROUND OF THE INVENTION[0002]Spread spectrum communication in its basic form is a method of taking a data signal that is used to modulate a sinusoidal carrier and then spreading its bandwidth to a much larger value, e.g. in a global positioning system (GPS) application, the spreading is achieved by multiplying a single-frequency carrier by a high-rate binary (−1,1) pseudo-random noise (PRN) code sequence known to GPS users. Thus, the signal that is transmitted includes a data component, a PRN component, and a (sinusoidal) carrier component.[0003]At the receiver of such a signal, a synchronized replica of the transmitted PRN code is required to de-spread the data sequence. Initial synchronization, called acquisition, is followed by fin...

AI Technical Summary

Benefits of technology

[0016]The invention provides fine frequency estimates of the carrier frequency used to convey ranging signals immediately after initial acquisition; such estimates can be used for tracking or as the initialization of more sensitive dine acquisition algorithm, and so simplifying the search for the carrier frequency. The invention thus allows removing a fine acquisition stage from a ranging receiver, saving in some embodiments for example as much as one or two seconds for acquisition in typical ranging receivers, or simplifying the operation of such a stage, reducing the computational burden in some embodiments for example by as much as a factor of up to fifty.

Problems solved by technology

The granularity of 10 Hz requires that the receiver search 1200×ks×1023 different code / frequency combinations and makes the sequential search so time-consuming as to be unrealistic, motivating the use of parallel and fast search methods.

An alternative is to provide a more precise signal acquisition, but to do so is problematic, because signal acquisition is based on performing a correlation of the received signal with a replica, a correlation that includes an estimate of the offset of the carrier frequency from a nominal carrier frequency (due to the Doppler shifting and clock inaccuracies mentioned above), and since the received signal is modulated not only by the PRN code (for example at 1.023 MHz for a coarse acquisition signal) but also by the navigation message at a bit rate of 50 Hz, the correlation becomes corrupted if a signal fragment is used that is longer than 20 ms (the duration of a navigation data bit).

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